Design Patterns Explained

While the solution I developed with the Bridge pattern is far superior
to the original solution, it is not perfect. One way of measuring
the quality of a design is to see how well it handles variation. Handling
a new implementation is very easy with a Bridge pattern in
place. The programmer simply needs to define a new concrete
implementation class and implement it. Nothing else changes.
However, things may not go so smoothly if I get a new concrete
example of the abstraction. I may get a new kind of Shape that can
be implemented with the implementations already in the design.
However, I may also get a new kind of Shape that requires a new
drawing function. For example, I may have to implement an
ellipse. The current Drawing class does not have the proper method
to do ellipses. In this case, I have to modify the implementations.
However, even if this occurs, I at least have a well-defined process
for making these changes (that is, modify the interface of the Drawing
class or interface, and modify each Drawing derivative accordingly)—this
localizes the impact of the change and lowers the risk
of an unwanted side effect.
Bottom line: Patterns do not always give perfect solutions. However,
because patterns represent the collective experience of many
designers over the years, they are often better than the solutions
you or I might come up with on our own.
In the real world, I do not always start out with multiple implementations.
Sometimes, I know that new ones are possible, but they
show up unexpectedly. One approach is to prepare for multiple
implementations by always using abstractions. You get a very
generic application.
Chapter 9 The Bridge Pattern 153
But I do not recommend this approach. It leads to an unnecessary
increase in the number of classes you have. It is important to write
code in such a way that when multiple implementations do occur
(which they often will), it is not difficult to modify the code to
The Bridge pattern
solution is good, but
not always perfect
Follow one rule, one
place to help with
refactoring